Fixed ma in calculate_gradient_across_cell_faces.
Fixed ma in calculate_gradient_across_cell_faces.

Fixed
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* Use the masked array in calculate_gradient_across_cell_faces. This
  fixes a merge error where the regular array was used in the subtract
  ufunc instead of the masked array.
* Fixed related doctests that used the masked arrays.

ManuSchmi88/landlab,laijingtao/landlab,Carralex/landlab,amandersillinois/landlab,SiccarPoint/landlab,amandersillinois/landlab,ManuSchmi88/landlab,Carralex/landlab,csherwood-usgs/landlab,Carralex/landlab,RondaStrauch/landlab,RondaStrauch/landlab,cmshobe/landlab,ManuSchmi88/landlab,cmshobe/landlab,landlab/landlab,decvalts/landlab,landlab/landlab,SiccarPoint/landlab,csherwood-usgs/landlab,RondaStrauch/landlab,laijingtao/landlab,cmshobe/landlab,landlab/landlab,decvalts/landlab